Light-emitting gallium nitride-based compound semiconductor device

ABSTRACT

A light-emitting gallium nitride-based compound semiconductor device of a double-heterostructure. The double-heterostructure includes a light-emitting layer formed of a low-resistivity In x Ga 1-x N (0&lt;x&lt;1) compound semiconductor doped with p-type and/or n-type impurity. A first clad layer is joined to one surface of the light-emitting layer and formed of an n-type gallium nitride-based compound semiconductor having a composition different from the light-emitting layer. A second clad layer is joined to another surface of the light-emitting layer and formed of a low-resistivity, p-type gallium nitride-based compound semiconductor having a composition different from the light-emitting layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a light-emitting galliumnitride-based compound semiconductor device and, more particularly, to alight-emitting compound semiconductor device having adouble-heterostructure capable of emitting high-power visible lightranging from near-ultraviolet to red, as desired, by changing thecomposition of a compound semiconductor constituting an active layer(light-emitting layer).

[0003] 2. Description of the Related Art

[0004] Gallium nitride-based compound semiconductors such as galliumnitride (GCN), gallium aluminum nitride (GaAlN), indium gallium nitride(InGaN), and indium aluminum gallium nitride (InAlGaN) have a directband gap, and their band gaps change in the range of 1.95 eV to 6 eV.For this reason, these compound semiconductors are promising asmaterials for light-emitting devices such as a light-emitting diode anda laser diode.

[0005] For example, as a light-emitting device using a gallium nitridesemiconductor, a blue light-emitting device in which a homojunctionstructure is formed on a substrate normally made of sapphire through anAlN buffer layer has been proposed. The homojunction structure includesa light-emitting layer formed of p-type impurity-doped GaN on an n-typeGaN layer. As the p-type impurity doped in the light-emitting layer,magnesium or zinc is normally used. However, even when the p-typeimpurity is doped, the GaN crystal has a poor quality, and remains ani-type crystal having a high resistivity almost close to an insulator.That is, the conventional light-emitting device is substantially of aMIS structure. As a light-emitting device having the MIS structure,layered structures in which Si- and Zn-doped, i-type GaAlN layers(light-emitting layers) are formed on n-type CaAlN layers are disclosedin Jpn. Pat. Appln. KOKAI Publication Nos. 4-10665, 4-10666, and4-10667.

[0006] However, in the light-emitting device having the MIS structure,both luminance and light-emitting output power are too low to bepractical.

[0007] In addition, the light-emitting device of a homojunction isimpractical because of the low power output by its nature. To obtain apractical light-emitting device having a large output power, it isrequired to realize a light-emitting device of a single-heterostructure,and more preferably, a double-heterostructure.

[0008] However, no light-emitting semiconductor devices of adouble-heterostructure are known, in which the double-heterostructure isentirely formed of low-resistivity gallium nitride-based compoundsemiconductors, and at the same time, has a light-emitting layerconsisting or low-resistivity, impurity-doped InGaN.

[0009] Jpn. Pat. Appln. KOKAI Publication Nos. 4-209577, 4-236477, and4-236478 disclose a light-emitting device having adouble-heterostructure in which an InGaN light-emitting layer issandwiched between an n-type InGaAlN clad layer and a p-type InGaAlNclad layer. However, the light-emitting layer is not doped with animpurity, and it is not disclosed or explicitly suggested that animpurity is doped into the light-emitting layer. In addition, the p-typeclad layer is a high-resistivity layer in fact. A similar structure isdisclosed in Jpn. Pat. Appln. KOKAI Publication No. 64-17484.

[0010] Jpn. Pat. Appln. KOKAI Publication 4-213878 discloses a structurein which an undoped InGaAlN light-emitting layer is formed on anelectrically conductive ZnO substrate, and a high-resistivity InGaNlayer is formed thereon.

[0011] Jpn. Pat. Appln. KOKAI Publication No. 4-68579 discloses adouble-heterostructure having a p-type GaInN clad layer formed on anoxygen-doped, n-type GaInN light-emitting layer. However, another cladlayer consists of electrically conductive ZnO. The oxygen is doped inthe light-emitting layer to be lattice-matched with the ZnO. Theemission wavelength of the light-emitting device having thisdouble-heterostructure is 365 to 406 nm.

[0012] All conventional light-emitting devices are unsatisfactory inboth output power and luminance, and have no satisfactory luminosity.

SUMMARY OF THE INVENTION

[0013] It is an object of the present invention to provide adouble-heterostructure in which all of the light-emitting layer (activelayer) and the clad layers are formed of low-resistivity galliumnitride-based III-V Group compound semiconductors, thereby realizing asemiconductor device exhibiting an improved luminance and/orlight-emitting output power.

[0014] It is another object of the present invention to provide alight-emitting device excellent in luminosity.

[0015] It is still another object of the present invention to provide anultraviolet to red light-emitting device having a wavelength in theregion of 365 to 620 nm.

[0016] According to the present invention, there is provided alight-emitting gallium nitride-based compound semiconductor devicehaving a double-heterostructure comprising:

[0017] a light-emitting layer (active layer) having first and secondmajor surfaces and formed of a low-resistivity In_(x)Ga_(1-x)N (0<x<1)compound semiconductor doped with an impurity;

[0018] a first clad layer joined to the first major surface of thelight-emitting layer and formed of an n-type gallium nitride-basedcompound semiconductor having a composition different from that of thecompound semiconductor of the light-emitting layer; and

[0019] a second clad layer joined to the second major surface of thelight-emitting layer and formed of a low-resistivity, p-type galliumnitride-based compound semiconductor having a composition different fromthat of the compound semiconductor of the light-emitting layer.

[0020] In the first embodiment, the compound semiconductor of thelight-emitting layer (active layer) is of p-type, doped with a p-typeimpurity.

[0021] In the second embodiment, the compound semiconductor of thelight-emitting layer (active layer) remains an n-type, doped with atleast a p-type impurity.

[0022] In the third embodiment, the compound semiconductor of thelight-emitting layer (active layer) is of n-type, doped with an n-typeimpurity.

[0023] In the present invention, the compound semiconductor of the firstclad layer is preferably represented by the following formula:

Ga_(y)A2_(1−y)N(0≦y≦1)

[0024] The compound semiconductor of the second clad layer is preferablyrepresented by the following formula:

Ga_(z)A2_(1−z)N(0≦z≦1)

[0025] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate presently preferredembodiments of the invention, and together with the general descriptiongiven above and the detailed description of the preferred embodimentsgiven below, serve to explain the principles of the invention.

[0027]FIG. 1 is a view showing a basic structure of a semiconductorlight-emitting diode of the present invention;

[0028]FIG. 2 is a graph showing a relationship between the lightintensity and the thickness of a light-emitting layer in thelight-emitting semiconductor device of the present invention;

[0029]FIG. 3 shows a photoluminescence spectrum of a low-resistivity,n-type In_(x)Ga_(1-x)N light-emitting layer according to the secondembodiment of the present invention;

[0030]FIG. 4 shows a photoluminescence spectrum of an undopedIn_(x)Ga_(1-x)N light-emitting layer;

[0031]FIG. 5 is a graph showing a relationship between a p-type impurityconcentration in the light-emitting layer and the light intensity in thelight-emitting semiconductor device according to the second embodimentof the present invention;

[0032]FIG. 6 is a graph showing a relationship between a p-type impurityconcentration in a p-type clad layer and the light emissioncharacteristics in the light-emitting semiconductor device according tothe second embodiment of the present invention;

[0033]FIG. 7 is a graph showing a relationship between an electroncarrier concentration in the light-emitting layer and the light emissioncharacteristics in the light-emitting semiconductor device according tothe second embodiment of the present invention;

[0034]FIG. 8 is a graph showing the light emission characteristics ofthe light-emitting semiconductor device according to the secondembodiment of the present invention;

[0035]FIG. 9 is a graph showing a relationship between an n-typeimpurity concentration in a light-emitting layer and the light emissioncharacteristics in a light-emitting semiconductor device according tothe third embodiment of the present invention;

[0036]FIG. 10 is a graph showing a relationship between a p-typeimpurity concentration in a p-type clad layer and the light emissioncharacteristics in the light-emitting semiconductor device according tothe third embodiment of the present invention;

[0037]FIG. 11 shows a structure of still another light-emitting diodeaccording to the present invention; and

[0038]FIG. 12 is a view showing a structure of a laser diode of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] The present invention provides a double-heterostructure in whichall of the light-emitting layer and clad layers sandwiching thelight-emitting layer are formed of low-resistivity gallium nitride-basedIII-V Group compound semiconductors, and at the same time, thelight-emitting layer is formed of an impurity-doped, low-resistivityIn_(x)Ga_(1-x)N compound semiconductor, thereby realizing a visiblelight emitting semiconductor device which is excellent in output power,luminance, and luminosity, for the first time.

[0040] The semiconductor device of the present invention includes alight-emitting diode (LED) and a laser diode (LD).

[0041] The present invention will be described below in detail withreference to the accompanying drawings. The same reference numeralsdenote the same parts throughout the drawings.

[0042]FIG. 1 shows a basic structure of an LED to which the presentinvention is applied. As shown in FIG. 1, an LED 10 of the presentinvention has a double-heterostructure 22 comprising a light-emittinglayer (active layer) 18 formed of impurity-doped, low-resistivity (LR)In_(x)Ga_(1-x)N, a first clad layer 16 joined to the lower surface(first major surface) of the light-emitting layer 18 and formed of ann-type, low-resistivity GaN-based III-V Group compound semiconductor,and a second clad layer 20 joined to the upper surface (second majorsurface) of the light-emitting layer 18 and formed of a p-type,low-resistivity GaN-based III-V Group compound semiconductor.In_(x)Ga_(1-x)N co the light-emitting layer 18 is a galliumnitride-based III-V Group compound semiconductor.

[0043] Because of the double-heterostructure, the compound semiconductorcomposition (except for impurities) of the first clad layer 16 isdifferent from that of the light-emitting layer 18. The compoundsemiconductor composition of the second clad layer 20 is also differentfrom that of the light-emitting layer 18. The compound semiconductorcompositions of the clad layers 16 and 20 may be the same or different.

[0044] The present inventors have made extensive studies on thelight-emitting device having all gallium nitride-based III-V Groupcompound semiconductor double-heterostructure having high light emissioncharacteristics, and found that, when the light-emitting layer is formedof In_(x)Ga_(1-x)N, and the ratio x of indium (In) is changed within therange of 0<x<1, a light-emitting device capable of emitting visiblelight ranging from near-ultraviolet to red can be obtained. The presentinventors have also found that, when an impurity is doped inIn_(x)Ga_(1-x)N and In_(x)Ga_(1-x)N has a low resistivity, alight-emitting device having improved light emission characteristics,especially a high output power, a high luminance, and a high luminositycould be obtained.

[0045] In the light-emitting device of the present invention, when thevalue of x in In_(x)Ga_(1-x)N of the light-emitting layer is close to 0,the device emits ultraviolet light. When the value of x increases, theemission falls in the longer-wavelength region. When the value of x isclose to 1, the device emits red light. When the value of x is in therange of 0<x<0.5, the light-emitting device of the present inventionemits blue to yellow light in the wavelength range of 450 to 550 nm.

[0046] In the present invention, an impurity (also called as a dopant)means a p- or n-type impurity, or both of them. In the presentinvention, the p-type impurity includes Group II elements such ascadmium, zinc, beryllium, magnesium, calcium, strontium, and barium. Asthe p-type impurity, zinc is especially preferable. The n-type impurityincludes Group IV elements such as silicon, germanium and tin, and GroupVI elements such as selenium, tellurium and sulfur.

[0047] In the present invention, “low-resistivity” means, when referredto a p-type compound semiconductor, that the p-type compoundsemiconductor has a resistivity of 1×10⁵ ω·m or less, and when referredto an n-type compound semiconductor, that the n-type compoundsemiconductor has a resistivity of 10 ω·cm or less.

[0048] Therefore, in the present invention, In_(x)Ga_(1-x)N of thelight-emitting layer 13 includes a low-resistivity, p-typeIn_(x)Ga_(1-x)N doped with a p-type impurity (the first embodiment to bedescribed below in detail), a low-resistivity, n-type In_(x)Ga_(1-x)Ndoped with at least a p-type impurity (the second embodiment to bedescribed below in detail), or an n-type In_(x)Ga_(1-x)N doped with ann-type impurity (the third embodiment to be described below in detail).

[0049] In the present invention, the first clad layer 16 is formed of alow-resistivity n-type gallium nitride-based III-V Group compoundsemiconductor. Although the n-type gallium nitride-based III-V Groupcompound semiconductor tends to be of an n-type even when undoped, it ispreferable to dope an n-type impurity therein and positively make ann-type compound semiconductor. The compound semiconductor forming thefirst clad layer 16 is preferably represented by the following formula:

Ga_(y)Al_(1-y)N(0≦y≦1)

[0050] In the present invention, the second clad layer 20 is formed of alow-resistivity, p-type gallium nitride-based III-V Group compoundsemiconductor doped with a p-type impurity. The compound semiconductoris preferably represented by the following formula:

Ga_(z)Al_(1-z)N(0≦z≦1)

[0051] The first, n-type clad layer 16 normally has a thickness of 0.05to 10 μm, and preferably has a thickness of 0.1 to 4 μm. An n-typegallium nitride-based compound semiconductor having a thickness of lessthan 0.05 tends not to function as a clad layer. On the other hand, whenthe thickness exceeds 10 μm, cracks tend to form in the layer.

[0052] The second, p-type clad layer 20 normally has a thickness of 0.05to 1.5 μm, and preferably has a thickness of 0.1 to 1 μm. A p-typegallium nitride-based compound semiconductor layer having a thicknessless than 0.05 μm tends to be hard to function as a clad layer. On theother hand, when the thickness of the layer exceeds 1.5 μm, the layertends to be difficult to be converted into a low-resistivity layer.

[0053] In the present invention, the light-emitting layer 18 preferablyhas a thickness within a range such that the light-emitting device ofthe present invention provides a practical relative light intensity of90% or more. In more detail, the light-emitting layer 18 preferably hasa thickness of 10 Å to 0.5 μm, and more preferably 0.01 to 0.2 μm. FIG.2 is a graph showing a measurement result of the relative lightintensities of blue light-emitting diodes each having the structureshown in FIG. 1. Each blue light-emitting diode was prepared by formingthe light-emitting layer 18 made of low-resistivity In_(0.1)Ga_(0.9)Nwhile changing the thickness. As is apparent from FIG. 2, when thethickness of the In_(x)Ga_(1-x)N light-emitting layer is 10 Å to 0.5 μm,the semiconductor device exhibits a practical relative light intensityof 90% or more. The almost same relationship between the thickness andthe relative light intensity was obtained for the low-resistivity p-typeIn_(x)Ga_(1-x)N doped with a p-type impurity, the low-resistivity,n-type In_(x)Ga_(1-x)N doped with at least a p-type impurity, and then-type In_(x)Ga_(1-x)N doped with an n-type impurity.

[0054] Referring back to FIG. 1, the double-heterostructure is normallyformed on a substrate 12 through an undoped buffer layer 14.

[0055] In the present invention, the substrate 12 can normally be formedof a material such as sapphire, silicon carbide (SiC), or zinc oxide(ZnO), and is most normally formed of sapphire.

[0056] In the present invention, the buffer layer 14 can be formed ofAlN or a gallium nitride-based compound semiconductor. The buffer layer14 is preferably formed of Ga_(m)Al_(1-m)N (0<m≦1). The Ga_(m)Al_(1-m)Nallows the formation of a gallium nitride-based compound semiconductor(first clad layer 16) having a better crystallinity thereon than on AlN.As is disclosed in U.S. patent application Ser. No. 07/826,997 filed onJan. 28, 1992 by Shuji NAKAMURA and assigned to the same assignee, theGa_(m)Al_(1-m)N buffer layer is preferably formed at a relatively lowtemperature of 200 to 900° C., and preferably 400 to 800° C. by themetaloranic chemical vapor deposition (MOCVD) method. The buffer layer14 preferably has substantially the same semiconductor composition asthe first clad layer 16 to be formed thereon.

[0057] In the present invention, the buffer layer 14 normally has athickness of 0.002 μm to 0.5 μm.

[0058] In the present invention, the first clad layer 16, thelight-emitting layer 18, and the second clad layer 20, all of whichconstitute the double-heterostructure, can be formed by any suitablemethod. These layers are preferably sequentially formed on the bufferlayer 14 by the MOCVD. The gallium source which can be used for theMOCVD includes trimethylgallium and triethylgallium. The indium sourceincludes trimethylindium and triethylindium. The aluminum sourceincludes trimethylaliminum and triethylaluminum. The nitrogen sourceincludes ammonia and hydrazine. The p-type dopant source includes GroupII compounds such as diethylcadmium, dimethylcadmium,cyclopentadienyl-magnesium, and diethylzinc. The n-type dopant sourceincludes Group IV compounds such as silane, and Group VI compounds suchas hydrogen sulfide and hydrogen selenide.

[0059] The gallium nitride-based III-V Group compound semiconductor canbe grown in the presence of the p-type impurity source and/or the n-typeimpurity source by using the above gas source at a temperature of 600°C. or more, and normally 1,200° C. or less. As a carrier gas, hydrogen,nitrogen or the like can be used.

[0060] In an as-grown state, the gallium nitride-based III-V Groupcompound semiconductor doped with a p-type impurity tends to exhibit ahigh resistivity and have no p-type characteristics (that is, it is nota low-resistivity semiconductor) even if the compound semiconductorcontains the p-type impurity. Therefore, as is disclosed in U.S. Ser.No. 07/970,145 filed on Nov. 2, 1992 by Shuji NAKAMURA, Naruhito IWASA,and Masayuki SENOH and assigned to the same assignee, the grown compoundsemiconductor is preferably annealed at a temperature of 400° C. ormore, and preferably 600° C. or more, for preferably one to 20 minutesor more, or the compound semiconductor layer is preferably irradiatedwith an electron beam while kept heated to a temperature of 600° C. ormore. When the compound semiconductor is annealed at such a hightemperature that the compound semiconductor may be decomposed, annealingis preferably performed in a compressed nitrogen atmosphere to preventthe decomposition of the compound semiconductor.

[0061] When annealing is performed, a p-type impurity in a form bondedwith hydrogen, such as Mg—H and Zn—H, is released from the bonds withthe hydrogen thermally, and the released hydrogen is discharged from thesemiconductor layer. As a result, the doped p-type impurityappropriately functions as an acceptor to convert the high-resistivitysemiconductor into a low-resistivity p-type semiconductor. Preferably,the annealing atmosphere does not therefore contain a gas containinghydrogen atoms (e.g., ammonia or hydrogen). Preferred examples of anannealing atmosphere includes nitrogen and argon atmospheres. A nitrogenatmosphere is most preferable.

[0062] After the double-heterostructure is formed, as shown in FIG. 1,the second clad layer 20 and the light-emitting layer 18 are partiallyetched away to expose the first clad layer 16. An n-electrode 24 isformed on the exposed surface while a p-electrode 26 is formed on thesurface of the first clad layer 20. The electrodes 24 and 26 arepreferably heat-treated to achieve ohmic contact to the semiconductorlayers. Above-described annealing may be achieved by this heattreatment.

[0063] The present invention has been generally described above. Thefirs, second, and third embodiments will be individually describedbelow. It should be understood that unique points of the respectiveembodiments will be particularly pointed out and explained, and theabove general description will be applied to these embodiments unlessotherwise specified, in the following description.

[0064] In the first embodiment of the present invention, low-resistivityIn_(x)Ga_(1-x)N constituting the light-emitting layer 18 of thedouble-heterojunction structure shown in FIG. 1 is of p-type, doped witha p-type impurity. Condition 0<x<0.5 is preferable to form thelight-emitting layer having a good crystallinity and obtain a blue toyellow light-emitting device excellent in the luminosity.

[0065] In the first embodiment, the concentration of the p-type impuritydoped in In_(x)Ga_(1-x)N of the light-emitting layer 18 should be higherthan the electron carrier concentration of a particular, correspondingundoped In_(x)Ga_(1-x)N (The electron carrier concentration of anundoped InGaN varies within a range of about 10¹⁷/cm³ to 1×10²²/cm³,depending on a particular growth condition used). Subject to thiscondition, the p-type impurity concentration is preferably about10¹⁷/cm³ to 1×10²¹/cm³ from the viewpoint of light emissioncharacteristics of the device. The most preferable p-type impurity iszinc. As described above, the p-type impurity-doped InGaN can beconverted into a low-resistivity InGaN by annealing (preferred) orradiating the electron beam.

[0066] In the second embodiment of the present invention, thelow-resistivity In_(x)Ga_(1-x)N constituting the light-emitting layer 18of the structure shown in FIG. 1 is of n-type, doped with at least ap-type impurity. Condition 0<x≦0.5 is preferable to provide thelight-emitting layer having a good crystallinity and obtain a blue toyellow light-emitting device excellent in the luminosity. In the secondembodiment, the light-emitting layer should be subjected to theannealing treatment described above, since it contains a p-typeimpurity.

[0067] In the second embodiment, when only a p-type impurity is doped inIn_(x)Ga_(1-x)N layer 18, the concentration of the p-type impurityshould be lower than the electron concentration of a correspondingundoped In_(x)Ga_(1-x)N. Subject to this condition, the p-type impurityconcentration is preferably 1×10¹⁶/cm³ to 1×10²²/cm³ from the viewpointof the light-emitting characteristics of the device. Especially, whenzinc is doped as the p-type impurity at a concentration of 1×10¹⁷/cm³ to1×10²¹/cm³, and especially 1×10¹⁸/cm³ to 1×10²⁰/cm³, the luminosity ofthe light-emitting device can be further improved and the luminousefficacy can be further increased.

[0068] In the second embodiment, the second clad layer 20 is asdescribed above. However, when magnesium is doped as the p-type impurityat a concentration of 1×10¹⁸/cm³ to 1×10²¹/cm³, the luminous efficacy ofthe light-emitting layer 18 can be further increased.

[0069]FIG. 3 is a diagram of the photoluminescence spectrum of a waferirradiated with a 10-mW laser beam from an He—Cd laser. The wafer wasprepared such that a low-resistivity In_(0.14)Ga_(0.86)N layer dopedwith cadmium (p-type impurity) was formed, according to the secondembodiment, or a GaN layer formed on a sapphire substrate. FIG. 4 is adiagram of the photoluminescence spectrum of a wafer prepared followingthe same procedures except that the In_(0.14)Ga_(0.86)N layer was notdoped with cadmium (undoped).

[0070] As can be apparent from FIG. 3, the p-type impurity-doped,low-resistivity In_(0.14)Ga_(0.86)N layer of the present inventionexhibits strong blue light emission near 480 nm. As can be apparent fromFIG. 4, undoped In_(0.14)Ga_(0.86)N layer not doped with a p-typeimpurity exhibits violet light emission near 400 nm. The same results asin FIG. 3 were obtained when zinc, beryllium, magnesium, calcium,strontium, and/or barium was doped, instead of Cd, according to thepresent invention. Thus, when the p-type impurity is doped in InGaNaccording to the present invention, the luminosity is improved.

[0071] When the p-type impurity is doped in InGaN, the photoluminescenceintensity can be greatly increased as compared to the undoped InGaN. Inthe device relating to FIG. 3, blue luminescence centers are formed inthe InGaN by the p-type impurity, thereby increasing the blueluminescence intensity FIG. 3 shows this phenomenon. In FIG. 3, a lowpeak appearing near 400 nm is the inter-band emission peak of theundoped In_(0.14)Ga_(0.86)N and corresponds to the peak in FIG. 4.Therefore, in the case of FIG. 3, the luminous intensity is increased by20 times or more as compared to FIG. 4.

[0072]FIG. 5 is a graph obtained by measuring and plotting the relativelight intensities and the Zn concentrations of blue light-emittingdevices each having the structure of FIG. 1. Each device was preparedsuch that the concentration of the p-type impurity Mg of the second cladlayer 20 was kept at 1×10²⁰/cm³, while changing the Zn concentration ofthe p-type impurity Zn-doped In_(0.1)Ga_(0.9)N of the light-emittinglayer 18. As shown in FIG. 5, the light-emitting device exhibits apractical relative intensity of 90% or more in the Zn concentrationrange of 1×10¹⁷/cm³ to 1×10²¹/cm³ and the highest relative lightintensity (almost 100%) in the Zn concentration range of 1×10¹⁸/cm³ to1×10²⁰/cm³.

[0073]FIG. 6 is a graph obtained by measuring and plotting the relativelight intensities and the Mg concentrations of blue light-emittingdevices each having the structure of FIG. 1. Each device was preparedsuch that the Zn concentration of the p-type impurity Zn-dopedIn_(0.1)Ga_(0.9)N of the light-emitting layer 18 was kept at 1×10²⁰/cm³,while changing the concentration of the p-type impurity Mg of the secondclad layer 20. As shown in FIG. 6, the light intensity of thelight-emitting device tends to rapidly increase when the Mgconcentration of the clad layer 20 exceeds 1×10¹⁷/cm³, and the lightintensity tends to rapidly decrease when the Mg concentration exceeds1×10²¹/cm³. FIG. 6 clearly shows that the light-emitting device exhibitsa practical relative intensity of 90% or more (almost 100%) when thep-type impurity concentration of the second clad layer 20 is in therange of 1×10¹⁸/cm³ to 1×10²¹/cm³. In FIGS. 5 and 6, the impurityconcentrations were measured by a secondary ion mass spectrometer(SIMS).

[0074] It is found that, more strictly, the electron carrierconcentration in the In_(x)Ga_(1-x)N layer is preferably in the range of1×10¹⁷/cm³ to 5×10²¹/cm³ when at least a p-type impurity is doped inIn_(x)Ga_(1-x)N to form an n-type In_(x)Ga_(1-x)N light-emitting layerhaving a low resistivity of 10 Ω·cm or less. The electron carrierconcentration can be measured by Hall effects measurements. When theelectron carrier concentration exceeds 5×10²¹/cm³, it is difficult toobtain a light-emitting device exhibiting a practical output power. Theelectron carrier concentration is inversely proportional to theresistivity. When the electron carrier concentration is less than1×10¹⁶/cm³, InGaN tends to be high-resistivity i-type InGaN, and theelectron carrier concentration cannot be measured. The impurity to bedoped may be only a p-type impurity, or both p- and n-type impurities.More preferably, both p- and n-type impurities are doped. In this case,zinc as the p-type impurity and silicon as the n-type impurity arepreferably used. Each of zinc and silicon is preferably doped at aconcentration of 1×10¹⁷/cm³ to 1×21/cm³. When the concentration of zincis lower than that of silicon, InGaN can be converted into preferablen-type InGaN.

[0075] When InGaN not doped with an impurity is grown, nitrogen latticevacancies are created to provide n-type InGaN. The residual electroncarrier concentration of this undoped n-type InGaN is about 1×10¹⁷/cm³to 1×10²²/cm³ depending on a growth condition used. By doping a p-typeimpurity serving as a luminescence center in the undoped n-type InGaNlayer, the electron carrier concentration in the n-type InGaN layer isdecreased. Therefore, when the p-type impurity is doped in InGaN suchthat the electron carrier concentration is excessively decreased, n-typeInGaN is converted into high-resistivity i-type InGaN. When the electroncarrier concentration is adjusted to fall within the above rangeaccording to the present invention, the output power is increased. Thisindicates that the p-type impurity serving as the luminescence centerperforms emission by forming donor-acceptor (D-A) light-emitting pairswith the donor impurity. The detailed mechanism has not been clarifiedyet. However, it is found that, in the n-type InGaN in which both donorimpurity (e.g., the n-type impurity or nitrogen lattice vacancy) formaking some electron carriers and the p-type impurity serving as anacceptor impurity are present, the light intensity by the formation ofthe luminescence centers is apparently increased. Since an increase inthe number of light-emitting pairs attributes to an increase in lightintensity as described, not only p-type impurity but also n-typeimpurity is preferably doped in InGaN. More specifically, when then-type impurity (especially silicon) is dosed in InGaN doped with thep-type impurity (especially zinc), the donor concentration is increased,and at the same time, a constant donor concentration with goodreproducibility can be obtained, unlike in undoped InGaN in which theelectron carrier concentration varies depending on the growth conditionas described above, and in which the donor concentration having aconstant residual concentration with good reproducibility is hardlyobtained. In fact, it is found that, by doping silicon, the electroncarrier concentration is increased from about 1×10¹⁸/cm³ to 2×10¹⁹/cm³by one figure, and the donor concentration is thus increased. Therefore,the amount of zinc to be doped can be increased by the increased amountof the donor concentration, and accordingly, the number of D-Alight-emitting pairs can be increased, thereby increasing the lightintensity.

[0076]FIG. 7 is a graph obtained by measuring and plotting the relativeoutput powers of blue light-emitting diodes and the elect-on carrierconcentrations in the InGaN layers (measured by Hall effectsmeasurements after growth of the InGaN layer). The blue light emittingdiode was prepared such that an Si-dozed n-type GaN layer was crown onthe sapphire substrate, a Zn-doped n-type In_(0.15)Ga_(0.85)N layer wasgrown thereon while changing the Zn concentration, and an Mg-dopedp-type GaN layer was grown. The points in FIG. 7 correspond to electroncarrier concentrations of 1×10¹⁶, 1×10¹⁷, 4×10¹⁷, 1×10¹⁸, 1×10¹⁹,4×10¹⁹, 1×10²⁰, 3×10²⁰, 1×10²¹, and 5×10²¹/cm³ from the left,respectively.

[0077] As shown in FIG. 7, the output power of the light-emitting devicechanges depending on the electron carrier concentration in the n-typeInGaN light-emitting layer. The output power starts to rapidly increaseat an electron carrier concentration of about 1×10¹⁶/cm³, reaches themaximum level at about 1×10¹⁹/cm³, slowly decreases until 5×10²¹/cm³,and rapidly decreases when the electron carrier concentration exceedsthat point. As is apparent from FIG. 7, when the electron carrierconcentration in the n-type InGaN layer is in the range of 1×10¹⁷/cm³ to5×10²¹/cm³, the light-emitting device exhibits an excellent outputpower.

[0078]FIG. 8 shows the light intensity when a laser beam from an He—Cdlaser was radiated on the n-type In_(0.15)Ga_(0.85)N layer doped withonly zinc at a concentration of 1×10¹⁸/cm³, and the n-typeIn_(0.15)Ga_(0.85)N layer doped with zinc and silicon at concentrationsof 1×10¹⁹/cm³ and 5×10¹⁹/cm³, respectively, and the photoluminescencewas measured at room temperature. The measurement result about then-type In_(0.15)Ga_(0.85)N layer doped with only zinc is represented bya curve a, and the measurement result about the n-typeIn_(0.15)Ga_(0.85)N layer doped with zinc and silicon is represented bya curve b (in the curve b, measured intensity is reduced to {fraction(1/20)}). Although the both InGaN layers exhibit the majorlight-emitting peaks at 490 nm, the n-type InGaN layer doped with bothzinc and silicon exhibits a light intensity ten times or more that ofthe n-type InGaN layer doped with only zinc.

[0079] In the third embodiment of the present invention, low-resistivityIn_(x)Ga_(1-x)N constituting the light-emitting layer 18 of thestructure of FIG. 1 is of n-type, doped with only an n-type impurity.Condition 0<x≦0.5 is preferable to provide a light-emitting layersemiconductor having a good crystallinity and obtain a bluelight-emitting device excellent in the luminosity.

[0080] In the third embodiment, the n-type impurity doped inIn_(x)Ga_(1-x)N of the light-emitting layer 18 is preferably silicon(Se). The concentration of the n-type impurity is preferably 1×10¹⁷/cm³to 1×10²¹/cm³ from the viewpoint of the light emission characteristics,and more preferably 1×10¹⁸/cm³ to 1×10²⁰/cm³.

[0081] In the third embodiment, as in the second embodiment, the secondclad layer 20 is as already described above. However, when magnesium isused as the p-type impurity, and is doped at a concentration of1×10¹⁸/cm³ to 1×10²¹/cm³, the luminous efficacy of the light-emittinglayer 18 can be further increased.

[0082]FIG. 9 is a graph obtained by measuring and plotting the relativelight intensities and the Si concentrations of blue light-emittingdevices each having the structure of FIG. 1. Each device was preparedsuch that the concentration of the p-type impurity Mg of the second cladlayer 20 was kept at 1×10¹⁹/cm³, while changing the Si concentration ofthe n-type impurity Si-doped In_(0.1)Ga_(0.9)N of the light-emittinglayer 18. As shown in FIG. 9, the light-emitting device exhibits apractical relative intensity of 90% or more in the Si concentrationrange of 1×10¹⁷/cm³ to 1×10²¹/cm³, and the highest relative lightintensity (almost 100%) in the Si concentration range of 1×10¹⁸/cm³ to1×10²⁰/cm³.

[0083]FIG. 10 is a graph obtained by measuring and plotting the relativelight intensities and the Mg concentrations of blue light-emittingdevices each having the structure of FIG. 1. Each device was preparedsuch that the Si concentration of the n-type impurity Si-dopedIn_(0.1)Ga_(0.9)N of the light-emitting layer 18 was kept at 1×10¹⁹/cm³,while changing the concentration of the p-type impurity Mg of the secondclad layer 20. As shown in FIG. 10, the light intensity of thelight-emitting device tends to rapidly increase when the Mgconcentration of the second p-type clad layer 20 exceeds 1×10¹⁷/cm³, andto rapidly decrease when the Mg concentration exceeds 1×10²¹/cm³. FIG.10 shows that the light-emitting device exhibits a practical relativeintensity of 90% or more (almost 100%) when the p-type impurityconcentration of the second clad layer 20 is in the range of 1×10¹⁸/cm³to 1×10²¹/cm³. In FIGS. 9 and 10, the impurity concentrations weremeasured by the SIMS.

[0084] In the third embodiment, the light-emitting device having thedouble-heterostructure of the present invention uses inter-band emissionof the n-type InGaN layer. For this reason, the half width of theemission peak is as narrow as about 25 nm, which is {fraction (1/2)} orless that of the conventional homojunction diode. In addition, thedevice of the present invention exhibits an output power four times ormore that of the homojunction diode. Further, when the value of x ofIn_(x)Ga_(1-x)N is changed in the range of 0.02<x<0.5, emission withinthe wavelength region of about 380 nm to 500 nm can be obtained asdesired.

[0085]FIG. 11 show a structure of a more practical light-emitting diode30 having a double-heterostructure of the present invention.

[0086] The light-emitting diode 30 a double-heterostructure 22constituted by an impurity-doped In_(x)Ga_(1-x)N light-emitting layer18, and two clad layers sandwiching the light-emitting layer 18, i.e.,an n-type gallium nitide-based compound semiconductor layer 16 and ap-type gallium nitride-based compound semiconductor layer 20, asdescribed above in detail.

[0087] A buffer layer 14 described above in detail is formed on asubstrate 20 described above in detail. An n-type GaN layer 32 is formedon the buffer layer 14 to a thickness of, for example, 4 to 5 μm, andprovides a contact layer for an n-electrode which is described below.The h-type contact layer 32 allows the formation of a clad layer 16having a better crystallinity, and can establish a better ohmic contactwith the n-electrode.

[0088] The double-heterostructure 22 is provided on the n-type contactlayer 32, with the clad layer 16 joined to the contact layer 32.

[0089] A p-type GaN contact layer 34 is formed on the clad layer 20 to athickness of, for example, 500 Å to 2 μm. The contact layer 34establishes a better ohmic contact with a p-electrode described below,and increases the luminous efficacy so the device.

[0090] The p-type contact layer 34 and the double-heterostructure 22 arepartially etched away to expose the n-type contact layer 32.

[0091] A p-electrode is provided on the p-type contact layer 34, and ann-electrode is provided on the exposed surface of the n-type contactlayer 32.

[0092] The light-emitting diodes embodying the present invention havebeen described above. However, the present invention should not belimited to these embodiments. The present invention encompasses varioustypes of light-emitting devices including a laser diode, so far as thosedevices have the double-heterostructures of the present invention.

[0093]FIG. 12 shows a structure of a laser diode 40 having adouble-heterostructure of the present invention.

[0094] The laser diode 40 has a double-heterostructure constituted by animpurity-doped In_(x)Ga_(1-x)N active layer 18 described above in detailin association with the light-emitting diode, and two clad layerssandwiching the active layer 18, i.e., an n-type gallium nitride-basedcompound semiconductor layer 16 and a p-type gallium nitride-basedcompound semiconductor layer 20, as described above. A buffer layer 14described above in detail is formed on a substrate 12 described above indetail. An n-type gallium nitride layer 42 is formed on the buffer layer14, providing a contact layer for an n-electrode described below.

[0095] The double-heterostructure 22 is provided on the n-type galliumnitride contact layer 42, with the clad layer joined to the contactlayer 42.

[0096] A p-type GaN contact layer 44 is formed on the clad layer 20.

[0097] The p-type contact layer 44, the double heterostructure 22 andpart of the n-type contact layer 42 are etched away to provide aprotruding structure as shown. A p-electrode is formed on the p-typecontact layer 44. A pair of n-electrodes 24 a and 24 b are formed on then-type GaN layer 42 to oppose each other, with the protruding structureintervening therebetween.

[0098] For example, the substrate 12 is a sapphire substrate having athickness of 100 μm, the buffer layer 14 is a GaN buffer layer having athickness of 0.02 μm, and the n-type GaN contact layer 42 has athickness of 4 μm. The first clad layer 16 is an n-type GaAlN clad layerhaving a thickness of 0.1 μm, the second clad layer 20 is a p-type GaAlNclad layer having a thickness of 0.1 μm, and the active layer 18 is ann-type layer doped with silicon or germanium. The p-type GaN contactlayer 44 has a thickness of 0.3 μm.

[0099] The present invention will be described below with reference tothe following examples. In the examples below, a compound semiconductorwas grown by the MOCVD method. An MOCVD apparatus used is a conventionalMOCVD apparatus having a structure in which a susceptor for mounting asubstrate thereon is arranged in a reaction vessel, and raw materialgases can be supplied together with a carrier gas toward a substratewhile the substrate is heated, thereby growing a compound semiconductoron the substrate.

EXAMPLE 1

[0100] Cleaning of Substrate:

[0101] First, a sapphire substrate sufficiently washed was mounted on asusceptor in an MOCVD reaction vessel, and the atmosphere in thereaction vessel was sufficiently substituted with hydrogen.Subsequently, while hydrogen was flown, the substrate was heated to1,050° C., and this temperature was held for 20 minutes, therebycleaning the sapphire substrate.

[0102] Growth of Buffer Layer:

[0103] The substrate was then cooled down to 510° C. While the substratetemperature was kept at 510° C., ammonia (NH₃) as a nitrogen source,trimethylgallium (TMG) as a gallium source, and hydrogen as a carriergas were kept supplied at flow rates of 4 liters (L)/min, 27×10⁻⁶mol/min, and 2 L/min, respectively, toward the surface of the sapphiresubstrate for one minute. Thus, a GaN buffer layer having a thickness ofabout 200 Å was grown on the sapphire substrate.

[0104] Growth of First Clad Layer:

[0105] After the buffer layer was formed, only the supply of TMG wasstopped, and the substrate was heated to 1,030° C. While the substratetemperature was kept at 1,030° C., the flow rate of TMG was switched to54×10⁻⁶ mol/min, silane gas (SiH₄) as an n-type impurity was added at aflow rate of 2×10⁻⁹ mol/min, and each material gas was supplied for 60minutes. Thus, an n-type GaN layer, doped with Si at a concentration of1×10²⁰/cm³, having a thickness of 4 μm was grown on the GaN bufferlayer.

[0106] Growth of Light-Emitting Layer:

[0107] After the first clad layer was formed, the substrate was cooleddown to 800° C. while flowing only the carrier gas. While the substratetemperature was kept at 800° C., the carrier gas was switched tonitrogen at a flow rate of 2 L/m-n, and TMG as a gallium source,trimethylindium (TMI) as an indium source, ammonia as a nitrogen source,and diethylcadmium as a p-type impurity source were supplied at flowrates of 2×10⁻⁶ mol/min, 1×10⁻⁵ mol/min, 4 L/min, and 2×10⁻⁶ mol/min,respectively, for ten minutes. Thus, an n-type In_(0.14)Ga_(0.86)Nlayer, doped with Cd at a concentration of 1×10²⁰/cm³, having athickness of 200 Å was grown on the first clad layer.

[0108] Growth of Second Clad Layer:

[0109] After the light-emitting layer was formed, the substrate washeated to 1,020° C. while flowing only the carrier gas nitrogen. Whilethe substrate temperature was kept at 1,020° C., the carrier gas wasswitched to hydrogen, a gallium source, TMG, a nitrogen source, ammonia,a p-type impurity source, cyclopentadienyl-magnesium (Cp₂Mg), weresupplied at flow rates of 54×10⁻⁶ mol/min, 4 L/min, 3.6×10⁻⁶ mol/min,respectively, for 15 minutes. Thus, a p-type GaN layer, doped with Mg ata concentration of 1×10²⁰/cm³, having a thickness of 0.8 μm was grown onthe light-emitting layer.

[0110] Conversion into Low-Resistivity Layer:

[0111] After the second clad layer was grown, the wafer was taken out ofthe reaction vessel. The wafer was annealed under nitrogen at atemperature of 700° C. or more for 20 minutes. Thus, the second cladlayer and the light-emitting layer were converted into low-resistivitylayers.

[0112] Fabrication of LED:

[0113] The second clad layer and the light-emitting layer of the waferobtained above were partially etched away to expose the first cladlayer. An ohmic n-electrode was formed on the exposed surface while anohmic p-electrode was formed on the second clad layer. The wafer was cutinto chips each having a size of 500 μm², and a blue light-emittingdiode was fabricated by a conventional method.

[0114] The blue light-emitting diode exhibited an output power of 300 μWat 20 mA, and its emission peak wavelength was 480 nm. The luminance ofthe light-emitting diode measured by a commercially available luminancemeter was 50 or more times that of a light-emitting diode of Example 5to be described later.

EXAMPLE 2

[0115] A blue light-emitting diode was prepared following the sameprocedures as in Example 1 except that, in the growth process of abuffer layer, trimethylaluminum (TMA) was used, instead of TMG, to forman AlN buffer layer on a sapphire substrate at a substrate temperatureof 600° C.

[0116] The blue light-emitting diode exhibited an output power of 80 μWat 20 mA, and its emission peak wavelength was 480 nm. The luminance ofthe light-emitting diode was about 20 times that of a light-emittingdiode of Example 5 to be described later.

EXAMPLE 3

[0117] Cleaning of a substrate and the growth of a buffer layer wereperformed following the same procedures as in Example 1.

[0118] A Her the buffer layer was formed, only the TMG flow was stopped,and the substrate was heated to 1,030° C. While the substratetemperature was kept at 1,030° C., and the flow rate of ammonia was notchanged, the flow rate of TMG was switched to 54×10⁻⁶ mol/min, and analuminum source, TMA, and a p-type impurity source, silane gas (SiH₄),were added at flow rates of 6×10⁻⁶ mol/min and 2×10⁻⁹ mol/min,respectively, and each gas was supplied for 30 minutes. Thus, an n-typeGa_(0.9)Al_(0.1)N layer (first clad layer), doped with Si at aconcentration of 1×10²⁰/cm³, having a thickness of 2 μm was grown on theGaN buffer layer.

[0119] A light-emitting layer was subsequently grown following the sameprocedures as in Example 1, to form a Cd-doped, n-typeIn_(0.14)Ga_(0.86)N layer having a thickness of 200 Å.

[0120] After the light-emitting layer was formed, supply of all the rawmaterial gases was stopped, and the substrate was heated to 1,020° C.While the substrate temperature was kept at 1,020° C., and the flow rateof the carrier gas was not changed, a gallium source, TMG, an aluminumsource, TMA, a nitrogen source, ammonia, and a p-type impurity source,Cp₂Mg, were supplied at flow rates of 54×10⁻⁶ mol/min, 6×10⁻⁶ mol/min, 4L/min, and 3.6×10⁻⁶ mol/min, respectively, for 15 minutes. Thus, ap-type Ga_(0.9)Al_(0.1)N layer (second clad layer) doped with Mg at aconcentration of 1×10²⁰/cm³, having a thickness of 0.8 μm was grown onthe light-emitting layer.

[0121] The annealing treatment and fabrication of a diode from the waferwere performed following the same procedures as in Example 1, to preparea blue light-emitting diode.

[0122] The blue light-emitting diode obtained above exhibited the sameoutput power, the same emission wavelength, and the same luminance as inthe diode of Example 1.

EXAMPLE 4

[0123] A blue light-emitting diode was prepared following the sameprocedures as in Example 1 except that, in the growth process of alight-emitting layer, Cp₂Mg was users instead of diethylcadmium at thesame flow rate to grow an Mg-doped, p-type In_(0.14)Ga_(0.86)Nlight-emitting layer.

[0124] The blue light-emitting layer obtained above exhibited the sameoutput power, the same emission wavelength, and the same luminance as inthe diode of Example 1.

EXAMPLE 5

[0125] A homojunction GaN light-emitting diode was prepared followingthe same procedures as in Example 1 except that no light-emitting InGaNlayer was grown.

[0126] The light-emitting diode exhibited an output power of 50 μW at 20mA. The emission peak wavelength was 430 nm, and the luminance was 2milicandela (mcd).

EXAMPLE 6

[0127] A blue light-emitting diode was prepared following the sameprocedures as in Example 1 except that, in the growth process of alight-emitting layer, silane gas at a flow rate of 2×10⁻⁹ mol/min wasused, instead of dimethylcadmlum, to form n-type In_(0.14)Ga_(0.86)Nlight-emitting layer doped with Si at a concentration of 1×10²⁰/cm³.

[0128] The light-emitting diode exhibited an output power output of 120μW at 20 mA. The emission peak wavelength was 400 nm, and the luminancewas about {fraction (1/50)} that of the diode in Example 1. The lowluminance was due to the short wavelength of the emission peak to lowerthe luminosity.

EXAMPLE 7

[0129] Cleaning of a substrate, the growth of a buffer layer, and thegrowth of a first clad layer (Si-doped, n-type GaN layer) were performedfollowing the same procedures as in Example 1.

[0130] After the first clad layer was formed, a light-emitting layer wasgrown as in Example 1 except that diethylzinc (DEZ) at a flow rate of1×10⁻⁶ mol/min was used, instead of diethylcadmium, to form an n-typeIn_(0.15)Ga_(0.85)N layer (light-emitting layer), doped with Zn at aconcentration of 1×10¹⁹/cm³, having a thickness of 200 Å on the firstclad layer.

[0131] A second clad layer was subsequently grown following the sameprocedures as in Example 1, to form an Mg-doped, p-type GaN layer havinga thickness of 0.8 μm. The annealing treatment and fabrication of adiode from the wafer were performed following the same procedures as inExample 1, to prepare a blue light-emitting diode.

[0132] The light-emitting device exhibited an output power of 300 μW at20 mA. The emission peak wavelength was 480 nm, and the luminance was400 mcd.

EXAMPLE 8

[0133] Cleaning of a substrate and the growth of a buffer layer wereperformed following the same procedures as in Example 1.

[0134] A first clad layer was grown following the same procedures as inExample 3, to form an Si-doped, n-type Ga_(0.9)Al_(0.1)N layer having athickness of 2 μm.

[0135] After the first clad layer was formed, a light-emitting layer wasgrown as in Example 7, to form an n-type In_(0.15)Ga_(0.85)N layer,doped with Zn at a concentration of 1×10¹⁹/cm³, having a thickness of200 Å.

[0136] After the light-emitting layer was formed, a second clad layerwas grown as in Example 3, to form a p-type Ga_(0.9)Al_(0.1)N layer,doped with Mg at a concentration of 1×10²⁰/cm³, having a thickness of0.8 μm on the light-emitting layer.

[0137] The annealing treatment of the second clad layer and fabricationof a diode from the wafer were performed following the same proceduresas in Example 1, to prepare a blud light-emitting diode.

[0138] The blue light-emitting diode obtained above exhibited the sameoutput power, the same emission peak wavelength, and the same luminanceas in the diode of Example 7.

EXAMPLE 9

[0139] A blue light-emitting diode was prepared following the sameprocedures as in Example 7 except that, in the growth process of alight-emitting layer, the flow rate of DEZ was increased, to form anIn_(0.15)Ga_(0.85)N light-emitting layer doped with zinc at aconcentration of 1×10²²/cm³.

[0140] The blue light-emitting diode thus obtained exhibited an outputpower of about 40% of that of the diode of Example 7.

EXAMPLE 10

[0141] A blue light-emitting diode was prepared following the sameprocedures as in Example 7 except that, in the growth process of asecond clad layer, the flow rate of Cp₂Mg was decreased, to form ap-type GaN layer (second clad layer) doped with Mg at a concentration of1×10¹⁷/cm³.

[0142] The light-emitting diode exhibited an output power of about 10%of that of the diode of Example 7.

EXAMPLE 11

[0143] Cleaning of a substrate, the growth of a buffer layer, and thegrowth of a first clad layer (Si-doped, n-type GaN layer) were performedfollowing the same procedures as in Example 1.

[0144] After the first clad layer was formed, a light-emitting layer wasgrown as in Example 1 except that diethylzinc was used, instead ofdiethycadimium, to form a Zn-doped, n-type In_(0.15)Ga_(0.85)N layerhaving a thickness of 100 Å on the first clad layer. The electroncarrier concentration of the n-type In_(0.5)Ga_(0.85)N layer was1×10¹⁹/cm³.

[0145] A second clad layer was grown following the same procedures as inExample 1, to form an Mg-doped, p-type GaN layer. The annealingtreatment and fabrication of a diode from the wafer were performed as inExample 1, to prepare a light emitting diode.

[0146] The light-emitting diode exhibited an output power of 400 μW at20 mA. The emission peak wavelength was 490 nm, and the luminance was600 mcd.

EXAMPLE 12

[0147] A blue light-emitting diode was prepared following the sameprocedures as in Example 11 except that, in the growth process of alight-emitting layer, the flow rate of DEZ gas was adjusted, to form ann-type In_(0.15)Ga_(0.85)N layer (light-emitting layer) having anelectron carrier concentration of 4×10¹⁷/cm³.

[0148] The light-emitting diode exhibited an output power of 40 μW at 20mA. The emission peak wavelength was 490 nm.

EXAMPLE 13

[0149] A blue light-emitting diode was prepared following the sameprocedures as in Example 11 except that, in the growth process of alight-emitting layer, the flow rate of the DEZ gas was adjusted, to forman n-type In_(0.15)Ga_(0.85)N layer (light-emitting layer) having anelectron carrier concentration of 1×10²¹/cm³.

[0150] The light-emitting diode exhibited an output power of 40 μW at 20mA The emission peak wavelength was 490 nm.

EXAMPLE 14

[0151] A blue light-emitting diode was prepared following the sameprocedures as in Example 11 except that, in the growth process of alight-emitting layer, the flow rate of the DEZ gas was adjusted, to forman n-type In_(0.15)Ga_(0.85)N layer (light-emitting layer) having anelectron carrier concentration of 1×10¹⁷/cm³.

[0152] The light-emitting diode exhibited an output power of 4 μW at 20mA. The emission peak wavelength was 490 nm.

EXAMPLE 15

[0153] A blue light-emitting diode was prepared following the sameprocedures as in Example 11 except that, in the growth process of alight-emitting layer, the flow rate of DEZ gas was adjusted, to form ann-type In_(0.15)Ga_(0.85)N layer having an electron carrierconcentration of 5×10²¹/cm³.

[0154] The light-emitting diode exhibited an output power of 4 μW at 20mA. The emission peak wavelength was 490 nm.

EXAMPLE 16

[0155] A buffer layer and an n-type GaN layer were formed on a sapphiresubstrate following the same procedures as in Example 11.

[0156] A high-resistivity, i-type GaN layer was grown by using TMG as agallium source, ammonia as a nitrogen source, and DEZ as a p-typeimpurity source. The i-type GaN layer was partially etched away toexpose the n-type GaN layer. An electrode was formed on the exposedsurface, and another electrode was formed on the i-type GaN layer,thereby preparing a light-emitting diode of a MIS structure.

[0157] The MIS structure diode exhibited a radiant power output of 1 μWat 20 mA and a luminance of 1 mcd.

EXAMPLE 17

[0158] A blue light-emitting diode was prepared following the sameprocedures as in Example 11 except that, in the growth process of alight-emitting layer, silane gas as an impurity source was added, toform an n-type In_(0.15)Ga_(0.85)N light-emitting layer, doped with Znand Si, having an electron carrier concentration of 1×10¹⁹/cm³.

[0159] The light-emitting diode exhibited an output power of 600 μW at20 mA. The emission peak wavelength was 490 nm, and the luminance was800 mcd.

EXAMPLE 18

[0160] Cleaning of a substrate, the growth of a buffer layer, and thegrowth of a first clad layer (Si-doped GaN layer) were performedfollowing the same procedures as in Example 1.

[0161] After the first clad layer was formed, a light-emitting layer wasgrown as in Example 1 except that silane and DEZ were used, instead ofdiethylcadmium, to form an n-type In_(0.14)Ga_(0.86)N layer, doped withSi and Zn, having a thickness of 100 Å on the first clad layer. Thelight-emitting layer had an electron carrier concentration of1×10¹⁸/cm³.

[0162] A second clad layer was grown following the same procedures as inExample 7, to form an Mg-doped (concentration of 2×10²⁰/cm³), p-type GaNlayer.

[0163] The annealing treatment and fabrication of an LED from the waferwere performed following the same procedures as in Example 1.

[0164] The blue light-emitting diode exhibited an output power of 580 μWat 20 mA. The luminance was 780 mcd, and the emission peak wavelengthwas 490 nm.

EXAMPLE 19

[0165] A blue light-emitting diode was prepared following the sameprocedures as in Example 18 except that, in the growth of alight-emitting layer, the flaw rates of the silane gas and the DEZ gas,were adjusted, to form an n-type In_(0.14)Ga_(0.86)N light-emittinglayer, doped with Si and Zn, having an electron carrier concentration of1×10²⁰/cm³.

[0166] The blue light-emitting diode exhibited an output power of 590 μWat 20 mA. The luminance was 790 mcd, and the emission peak wavelengthwas 490 nm.

EXAMPLE 20

[0167] A blue light-emitting diode was prepared following the sameprocedures as in Example 18 except that, in the growth process of alight-emitting layer, the flow rates of the silane gas and the DEZ gaswere adjusted, to form an n-type In_(0.14)Ga_(0.86)N light-emittinglayer, doped with Si and Zn, having an electron carrier concentration of4×10¹⁷/cm³.

[0168] The blue light-emitting diode exhibited a radiant power output of60 μW at 20 mA. The luminance was 80 mcd, and the emission peakwavelength was 490 nm.

EXAMPLE 21

[0169] A blue light-emitting diode was prepared following the sameprocedures as in Example 18 except that, in the growth process of alight-emitting layer, the flow rates of the silane gas and the DEZ gaswere adjusted, to form an n-type In_(0.14)Ga_(0.86)N light-emittinglayer, doped with Si and Zn, having an electron carrier concentration of5×10²¹/cm³.

[0170] The blue light-emitting diode exhibited an output power of 6 μWat 20 mA. The luminance was 10 mcd, and the emission peak wavelength was490 nm.

EXAMPLE 22

[0171] A green light-emitting diode was prepared following the sameprocedures as in Example 18 except that, in the growth process of alight-emitting layer, the flow rate of TMI was adjusted, to form an Si-and Zn-doped In_(0.25)Ga_(0.75)N light-emitting layer.

[0172] The green light-emitting layer exhibited an output power of 500μW at 20 mA. The luminance was 1,000 mcd, and the emission peakwavelength was 510 nm.

EXAMPLE 23

[0173] A buffer layer and an n-type GaN layer were formed a sapphiresubstrate following the same procedures as in Example 11.

[0174] Using TMG as a gallium source, ammonia as a nitrogen source, andsilane and DEZ as impurity sources, an i-type GaN layer doped with Siand Zn was formed. The i-type GaN layer was partially etched away toexpose the n-type GaN layer. An electrode was formed on the exposedsurface, and another electrode was formed on the i-type GaN layer,thereby preparing a light-emitting diode of a MIS structure.

[0175] The MIS structure diode exhibited an output power of 1 μW at 20mA, and a luminance of 1 mcd.

EXAMPLE 24

[0176] Cleaning of a substrate, the growth of a buffer layer, and thegrowth of a first clad layer (Si-doped, n-type GaN layer) were performedfollowing the same procedures as in Example 1.

[0177] After the first clad layer was formed, a light-emitting layer wasgrown as in Example 1 except that an n-type impurity source, silane, wasused, instead of diethylcadmium, at an adjusted flow rate, and growthwas conducted for 5 minutes, to form an n-type In_(0.15)Ga_(0.85)Nlight-emitting layer, doped with Si at a concentration of 1×10²⁰/cm³,having a thickness of 100 Å on the first clad layer.

[0178] Then, a second clad layer was grown as in Example 1 except thatthe flow rate of Cp₂Mg was adjusted, to form a p-type GaN layer (secondclad layer) doped with Mg at a concentration of 1×10¹⁸/cm³. Theannealing treatment and fabrication of a diode from the wafer wereperformed as in Example 1, to prepare a blue light-emitting diode.

[0179] The light-emitting diode exhibited an output power of 300 μW at20 mA. The emission peak wavelength was 405 nm.

EXAMPLE 25

[0180] A blue light-emitting diode was prepared following the sameprocedures as in Example 24 except that, in the growth process of afirst clad layer, an Si-doped, n-type Ga_(0.9)Al_(0.1)N layer (firstclad layer) having a thickness of 2 fm was formed following the sameprocedures as in Example 3, and in the growth process of a second cladlayer, a p-type Ga_(0.9)Al_(0.1)N layer (second clad layer), doped withMg at a concentration of 1×10¹⁸/cm³, having a thickness of 0.8 μm wasformed following the same procedures as in Example 3.

[0181] The light-emitting diode exhibited the same output power and thesame emission peak wavelength as in the light-emitting diode of Example24.

EXAMPLE 26

[0182] A blue light-emitting diode was prepared following the sameprocedures as in Example 24 except that, an the growth process of alight-emitting layer, the flow rate of silane gas was increased, to forman n-type In_(0.15)Ga_(0.85)N layer doped with Si at a concentration of1×10²²/cm³.

[0183] The output of the light-emitting diode was about. 40% of that ofthe diode of Example 24.

EXAMPLE 27

[0184] A blue light-emitting diode was prepared following the sameprocedures as in Example 24 except that, in the growth process of asecond clad layer, the flow rate of Cp₂Mg was decreased, to form ap-type GaN layer doped with Mg at a concentration of 1×10¹⁷/cm³.

[0185] The output of the light-emitting diode was about 20% of that ofthe diode of Example 24.

EXAMPLE 28

[0186] Cleaning of a substrate and the growth of a buffer layer wereperformed following the same procedures as in Example 1.

[0187] After the buffer layer was formed, only the TMG flow was stopped,and the substrate was heated to 1,030° C. While the substratetemperature was kept at 1,030° C., and the flow rate of ammonia was notchanged, the flow rate of TMG was switched to 54×10⁻⁶ mol/min, an n-typeimpurity source, silane, was added at a flow rate of 2×10⁻⁹ mol/min, andthe growth was conducted for 60 minutes. Thus, n-type GaN layer (n-typecontact layer), doped with Si at a concentration of 1×10²⁰/cm³, having athickness of 4 μm was formed or the GaN buffer layer.

[0188] Then, an aluminum source, TMA, at an adjusted flow rate wasadded, and the growth was conducted in a similar manner to that inExample 3, to form an Si-doped n-type Ga_(0.8)Al_(0.2)N layer (firstclad layer) having a thickness of 0.15 μm on the n-type contact layer.

[0189] Next, a light-emitting layer was grown in the same procedures asin Example 17, to form an n-type In_(0.14)Ga_(0.86)N light-emittinglayer, doped with Si and Zn, having an electron carrier concentration of1×10¹⁹/cm³ on the first clad layer.

[0190] Subsequently, a second clad layer was grown for 2 minutes in asimilar manner to that Example 3, to form an Mg-doped Ga_(0.8)Al_(0.2)Nlayer having a thickness of 0.15 μm on the light-emitting layer.

[0191] Then, only the aluminum source flow was stopped, and the growthwas conducted for 7 minutes, to form an Mg-doped GaN layer (p-typecontact layer) having a thickness of 0.3 μm on the second clad layer.

[0192] The annealing treatment was conducted as in Example 1, to convertthe light-emitting layer, the second clad layer and the p-type contactlayer into low-resistivity layers.

[0193] From the wafer, a light-emitting diode having a structure of FIG.11 was fabricated.

[0194] This diode exhibited an output power of 700 μW and a luminance of1,400 mcd. The emission peak wavelength was 490 nm. The forward voltagewas 3.3V at 20 mA.

[0195] This forward voltage was about 4V lower than that of the diode ofExample 3, 8 or 25. This lower forward voltage is due to the betterohmic contact between the GaN contact layers and the electrodes.

[0196] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details, and representativedevices shown and described herein. Accordingly, various modificationsmay be made without departing from the spirit or scope of the generalinventive concept as defined by the appended claims and theirequivalents.

What is claimed is:
 1. A light-emitting gallium nitride-based compoundsemiconductor device having a double-heterostructure comprising: alight-emitting layer having first and second major surfaces and formedof a low-resistivity In_(x)Ga_(1-x)N, where 0<x<1, compoundsemiconductor doped with an impurity; a first clad layer joined to saidfirst major surface of said light-emitting layer and formed of an n-typegallium nitride-based compound semiconductor having a compositiondifferent from that of said compound semiconductor of saidlight-emitting layer; and a second clad layer joined to said secondmajor surface of said light-emitting layer and formed of alow-resistivity, p-type gallium nitride-based compound semiconductorhaving a composition different from that of said compound semiconductorof said light-emitting layer.
 2. The device according to claim 1,wherein said compound semiconductor of said light-emitting layer is ofp-type, doped with a p-type impurity.
 3. The device according to claim2, wherein said p-type impurity comprises a Group II element.
 4. Thedevice according to claim 1, wherein said compound semiconductor of saidlight-emitting layer is of n-type, doped with at least a p-conductivitytype impurity.
 5. The device according to claim 3, wherein said impuritydoped in said compound semiconductor of said light-emitting layercomprises a p-type impurity including a Group II element and an n-typeimpurity including a Group IV or VI element.
 6. The device according toclaim 1, wherein said compound semiconductor of said light-emittinglayer is of n-type, does with an n-type impurity.
 7. The deviceaccording to claim 6, wherein said n-type impurity comprises a Group IVor VI element.
 8. The device according to claim 1, wherein said compoundsemiconductor or said first clad layer is represented by the formula:Ga_(y)Al_(1-y)N, where 0≦y≦1.
 9. The device according to claim 1,wherein said compound semiconductor of said second clad layer isrepresented by the formula: Ga_(z)Al_(1-z)N, where 0≦z≦1.
 10. The deviceaccording to claim 1, wherein said light-emitting layer has a thicknessof 10 Å to 0.5 μm.
 11. The device according to claim 1, wherein saiddouble-heterostructure has an n-type GaN contact layer joined to saidfirst clad layer, and a p-type GaN contact layer joined to said secondclad layer.
 12. The device according to claim 1, wherein 0<x<0.5.
 13. Alight-emitting gallium nitride-based compound semiconductor devicehaving a double-heterostructure comprising: a light-emitting layerhaving first and second major surfaces and formed of a low-resistivityIn_(x)Ga_(1-x)N, where 0<x<1, compound semiconductor doped with a p-typeimpurity; a first clad layer joined to said first major surface of saidlight-emitting layer and formed of an n-type gallium nitride-basedcompound semiconductor having a composition different from that of saidcompound semiconductor of said light-emitting layer; and a second cladlayer joined to said second major surface of said light-emitting layerand formed of a low-resistivity, p-type gallium nitride-based compoundsemiconductor having a composition different from that of said compoundsemiconductor of said light-emitting layer.
 14. The device according toclaim 13, wherein said p-type impurity doped in said compoundsemiconductor of said light-emitting layer comprises at least oneelement selected from the group consisting of cadmium, zinc, beryllium,magnesium, calcium, strontium, and barium.
 15. The device according toclaim 13, wherein said compound semiconductor of said first clad layeris represented by a formula: Ga_(y)Al_(1-y)N, where 0≦y≦1.
 16. Thedevice according to claim 13, wherein said compound semiconductor ofsaid second clad layer is represented by a formula: Ga_(z)Al_(1-z)N,where 0≦z≦1.
 17. The device according to claim 13, wherein saidlight-emitting layer has a thickness of 10 Å to 0.5 μm.
 18. The deviceaccording to claim 13, wherein said p-type impurity doped in saidcompound semiconductor of said light-emitting layer comprises zinc, anda concentration of the zinc is 1×10¹⁷ to 1×10²¹/cm³.
 19. The deviceaccording to claim 13, wherein said p-type impurity doped in saidcompound semiconductor of said second clad layer comprises magnesium,and a concentration of the magnesium is 1×10¹⁸ to 1×10²¹/cm³.
 20. Thedevice according to claim 13, wherein said second clad layer has athickness of 0.05 μm to 1.5 μm.
 21. The device according to claim 13,wherein said double-heterostructure is provided on a substrate through abuffer layer.
 22. The device according to claim 13, wherein saiddouble-heterostructure has an n-type GaN contact layer joined to saidfirst clad layer, and a p-type GaN contact layer joined to said secondclad layer.
 23. The device according to claim 13, wherein 0<x<0.5
 24. Alight-emitting gallium nitride-based compound semiconductor devicehaving a double-heterostructure comprising: a light-emitting layerhaving first and second major surfaces and formed of a low-resistivity,n-type In_(x)Ga_(1-x)N, where 0<x<1, compound semiconductor doped withat least a p-type impurity; a first clad layer joined to said firstmajor surface of said light-emitting layer and formed of an n-typegallium nitride-based compound semiconductor having a compositiondifferent from that of said compound semiconductor of saidlight-emitting layer; and a second clad layer joined to said secondmajor surface and formed of a low-resistivity, p-type galliumnitride-based compound semiconductor having a composition different fromthat of said compound semiconductor of said light-emitting layer. 25.The device according to claim 24, wherein said compound semiconductor ofsaid light-emitting layer has an electron carrier concentration of1×10¹⁷ to 5×10²¹/cm³.
 26. The device according to claim 24, wherein saidcompound semiconductor of said light-emitting layer is doped with notonly said p-type impurity but also an n-type impurity.
 27. The deviceaccording to claim 24, wherein said p-type impurity doped in saidcompound semiconductor of said light-emitting layer comprises at leastone element selected from the group consisting of cadmium, zinc,beryllium, magnesium, calcium, strontium, and barium.
 28. The deviceaccording to claim 26, wherein said n-type impurity doped in saidcompound semiconductor of said light-emitting layer comprises at leastone element selected from the group consisting of silicon, germanium,and tin.
 29. The device according to claim 24, wherein said compoundsemiconductor of said first clad layer is represented by a formula:Ga_(y)Al_(1-y)N, where 0≦y≦1.
 30. The device according to claim 24,wherein said compound semiconductor of said second clad layer isrepresented by a formula: Ga_(z)Al_(1-z)N, where 0≦z≦1.
 31. The deviceaccording to claim 26, wherein said p-type impurity doped in saidcompound semiconductor of said light-emitting layer comprises zinc, andsaid n-type impurity comprises silicon.
 32. The device according toclaim 24, wherein said double-heterostructure is provided on a substratethrough a buffer layer.
 33. The device according to claim 24, whereinsaid double-heterostructure has an n-type GaN contact layer joined tosaid first clad layer, and a p-type GaN contact layer joined to saidsecond clad layer.
 34. The device according to claim 24, wherein0<x<0.5.
 35. A light-emitting gallium nitride-based compoundsemiconductor device having a double-heterostructure comprising alight-emitting layer having first and second major surfaces and formedof a low-resistivity, n-type In_(x)Ga_(1-x)N, where 0<x<1, compoundsemiconductor doped with an n-type impurity; a first clad layer joinedto said first major surface of said light-emitting layer and formed ofan n-type gallium nitride-based compound semiconductor having acomposition different from that of said compound semiconductor of saidlight-emitting layer; and a second clad layer joined to said secondmajor surface of said light-emitting layer and formed so alow-resistivity, p-type gallium nitride-based compound semiconductorhaving a composition different from that of said compound semiconductorof said light-emitting layer.
 36. The device according to claim 35,wherein said n-type impurity doped in said compound semiconductor ofsaid light-emitting layer comprises silicon or germanium.
 37. The deviceaccording to claim 35, wherein said n-type impurity doped in saidcompound semiconductor of said light-emitting layer comprises silicon,and a concentration of the silicon is 1×10¹⁷ to 1×10²¹/cm³.
 38. Thedevice according to claim 35, wherein said compound semiconductor ofsaid first clad layer is represented by a formula: Ga_(y)Al_(1-y)N,where 0≦y≦1.
 39. The device according to claim 35, wherein said compoundsemiconductor of said second clad layer is represented by a formula:Ga_(z)Al_(1-z)N, where 0≦z≦1.
 40. The device according to claim 35,wherein said light-emitting layer has a thickness of 10 Å to 0.5 μm. 41.The device according to claim 35, wherein said compound semiconductor ofsaid second clad layer is doped with a p-type impurity comprisingmagnesium, and a concentration of the magnesium is 1×10¹⁸ to 1×10²¹/cm³.42. The device according to claim 35, wherein said second clad layer hasa thickness of 0.05 to 1.5 μm.
 43. A device according to claim 35,wherein said double-heterostructure is provided on a substrate through abuffer layer.
 44. The device according to claim 35, wherein saiddouble-heterostructure has an n-type GaN contact layer joined to saidfirst clad layer, and a p-type GaN con-tact layer joined to said secondclad layer.
 45. The device according to claim 35, wherein 0<x<0.5.